I wanna build my first 20" Dobsonian

[Stargazer10]

I've bought an 8inch Newtonian from Omegon with push+ but it's inaccurate and the magnification isn't great.
Since I don't have the money for a new one, I wanna build a DIY scope from scratch. I've seen several videos and read about it.
Do you have any tips for me?
I'm planning to polish my own mirror after melting multiple layers of glass in a kiln.

Many thanks!!!

[Davey-T]

Sounds like a long job, think you need to spin the molten glass and control the cooling but some of the original big professional scope mirrors were made from melted down beer bottles.

Dave

[Peter Drew]

Welcome to SGL Stargazer10.

Whilst I would be one of the last to deter anyone from embarking on an ambitious project, I think you will have to be very aware of the complexity of mirror making let alone mirror casting.  The former takes a great deal of prior experience on smaller mirrors and the latter is a science in itself bearing in mind the accuracy of the annealing process which can take several days.

In what way was the magnification of your 8" "not great"?.  I have an 8" and a 20" and on most nights in the UK the maximum magnification possible with either is very similar, it's the atmosphere that's the limitation, not the aperture.    🙂  

[Piero]

To me most of the time, the "atmosphere" 1 inch on and above the mirror related to improper cooling, uneven mirror temperature in general, poor mechanics (e.g. mirror back and lateral support, the way the secondary mirror is mounted, the way the focuser drawtube moves, etc), often so and so optics, and poor collimation are at the basis of limited magnifications in larger telescopes, rather than the actual atmosphere.

To the OP. That's a great project. One warning though as you mentioned finances. Even if you make the whole thing, including the mirrors, the overall cost will still be noticeable.

[Stargazer10]

21 hours ago, Peter Drew said:

Welcome to SGL Stargazer10.

Whilst I would be one of the last to deter anyone from embarking on an ambitious project, I think you will have to be very aware of the complexity of mirror making let alone mirror casting.  The former takes a great deal of prior experience on smaller mirrors and the latter is a science in itself bearing in mind the accuracy of the annealing process which can take several days.

In what way was the magnification of your 8" "not great"?.  I have an 8" and a 20" and on most nights in the UK the maximum magnification possible with either is very similar, it's the atmosphere that's the limitation, not the aperture.    🙂  

Thanks for the response. 

I'm aware of the complexity of that task and I've bought three  5 inch glass blankets for 'training'. Hopefully one day I will be able to make a serviceable device.

As for the magnification, a 203/1000 scope isn't enough for planets. I'm already using a barlow 2x and what I get is 222x (9mm eyepiece) what makes a planet like saturn or jupiter look quite small. I guess a magn. of 1000 is needed to get a satisfying view. 

[Stargazer10]

12 hours ago, Piero said:

To me most of the time, the "atmosphere" 1 inch on and above the mirror related to improper cooling, uneven mirror temperature in general, poor mechanics (e.g. mirror back and lateral support, the way the secondary mirror is mounted, the way the focuser drawtube moves, etc), often so and so optics, and poor collimation are at the basis of limited magnifications in larger telescopes, rather than the actual atmosphere.

To the OP. That's a great project. One warning though as you mentioned finances. Even if you make the whole thing, including the mirrors, the overall cost will still be noticeable.

Thanks for the response. 

It's a new scope, so I assume it doesn't have to be collimated. But what do I even know. I'm an amateur willing to learn.

[John]

The problem is, even with a very large aperture scope, magnifications of 300x - 500x are not useful other than very rarely because of the effects that observing through our atmosphere has.

As a device to observe very faint objects, at somewhat lower magnifications, 20 inches is an amazing amount of light gathering power.

Edited November 18, 2020 by John

[cloudsweeper]

   

11 minutes ago, Stargazer10 said:

Thanks for the response. 

It's a new scope, so I assume it doesn't have to be collimated. But what do I even know. I'm an amateur willing to learn.

All Newts/Dobs need to be collimated, and have it checked fairly regularly.  And a 1000mm focal length should be enough for planets (depending on the eyepiece used).  You mentioned a mag of x1000 - this is not going to happen.  Turbulence limits mag to say x200, up to maybe x400 under very good conditions.  With magnification, it is often a case of less is more.

Doug.

Edited November 18, 2020 by cloudsweeper

[Piero]

I use 300-500x quite regularly after optimising my dobson and 150-370x with my refractor, so I cannot agree with the statements above.